"How many dollars do you have?"
Translation:Kiom da dolaroj vi havas?
Ĉu iun ideon, kial la akuzativo ne estis uzita ĉi tie? (Maybe you could also write "Ĉu iu ideo...", but I think "iun ideon" is better because it seems to me that the whole sentence is "Ĉu iu [el vi] havas iun ideon...")
Yes, you would really say "Mi havas 10 dolarojn", but here it's "Kiom da dolaroj". Da is a preposition, so you don't need (and aren't allowed) to use the accusative here. You only use the accusative after a preposition if it means movement. For example:
- Birdo flugas en mia domo = A bird is flying in my house (it's in my house and it's flying there)
- Birdo flugas en mian domon = A bird is flying into my house (it's flying from outside my house into it)